Heat pump on c2 splitter bottom to unload propylene refrigeration

ABSTRACT

A method of cooling an ethylene discharge gas includes the steps of drawing a liquid ethane from a C 2  splitter; reducing a pressure of the drawn liquid ethane in a let-down valve to produce a cooled liquid ethane; cooling the ethylene discharge gas with the cooled liquid ethane in a vaporizer, the cooled liquid ethane exiting the vaporizer as an ethane vapor; pressurizing the ethane vapor in a heat pump to produce a heated vapor ethane; and returning the heated vapor ethane to the C 2  splitter. The ethylene discharge gas may be from an ethylene refrigerant compressor.

BACKGROUND

1. Field of the Disclosure

The present disclosure generally relates to systems and methods for providing refrigeration service.

2. Description of the Related Art

Ethylene is a building block in the manufacturing of a wide variety of chemical materials and is typically produced industrially by converting hydrocarbons in a cracking furnace or a catalytic reactor (such as MTO reactor, Superflex, or others), generally in the presence of steam. The furnace or reactor effluent stream comprising a range of components is typically cleaned up, dried to remove water, compressed and passed to an olefins recovery section to separate the ethylene from other light hydrocarbons, such as ethane, propylene, propane, and the like. One of the steps to produce purified ethylene generally uses a C2-splitter (distillation tower) to separate ethylene from an ethylene-ethane mixture. The present disclosure relates to reducing refrigeration power for the processes used to manufacture ethylene and other chemicals by using the C2-splitter reboiler duty more efficiently.

SUMMARY

In aspects, the present disclosure provides a method of cooling a process stream. The method may include the steps of drawing a liquid ethane from a C2 splitter; reducing a pressure of the drawn liquid ethane in a let-down valve to produce a cooled liquid ethane; cooling the process stream with the cooled liquid ethane in a vaporizer, the cooled liquid ethane exiting the vaporizer as an ethane vapor; pressurizing the ethane vapor in a heat pump and returning the compressed vapor ethane to the C2 splitter. The process stream may be gas from an ethylene refrigerant compressor or any other process stream.

It should be understood that examples of certain features of the disclosure have been summarized rather broadly in order that the detailed description thereof that follows may be better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the disclosure that will be described hereinafter and which will in some cases form the subject of the claims appended thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

For detailed understanding of the present disclosure, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein:

FIG. 1 depicts a system for cooling an ethylene discharge gas according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown a system 10 for providing refrigeration to condense an ethylene discharge gas 12 from an ethylene refrigerant compressor 14. In the non-limiting embodiment shown, the system 10 includes a C2 splitter 18, a line 20 for receiving a liquid ethane 16 (bottoms) from the C2 splitter 18, a let-down valve 22 connected to the line 20, an ethane vaporizer 24 downstream of the let-down valve 22, a line 26 from the ethane vaporizer 24, a heat pump 28 connected to the line 26, and a flow line 30 conveying the ethane from the heat pump 28 to the C2 splitter 18. As described below, the system 10 uses the heat pump 28 with the bottoms of the C2 splitter 18 as a working fluid for providing refrigeration for ethylene refrigeration discharge condensing.

In one mode of operation, the liquid ethane 16 is drawn from the C2 splitter 18 and conveyed to the let-down valve 22 via the line 20. The C2 splitter 18 may be any conventional device for the separation of ethylene from ethane by distillation. The let-down valve 22 drops the pressure of the liquid ethane 16. The let-down valve 22 may be any device that reduces or steps down the pressure in a fluid. The vaporizer 24 may be any device that allows indirect heating between two or more fluids (e.g., the fluids do not directly contact one another but are separated by a heat transmitting wall). In the vaporizer 24, the liquid ethane 16 cools and/or condenses the ethylene 12. The ethane 16 exits the vaporizer 24 as a vapor. The line 26 conveys the vaporized ethane 16 to the heat pump 28. The heat pump 28 compresses the ethane 16 and the line 30 conveys the compressed ethane 16 to the C2 splitter 18. The heat pump may be any device to increase the pressure of ethane vapor from line 26.

By way of illustration only, the liquid ethane 16 drawn from the C2 splitter 18 may be at 360 PSIA (P1) and 34 degrees F. (Ti). After having the pressure let down at the let-down valve 22 and flowing through the ethane feed vaporizer 24, the ethane 16 may be at 135 PSIA (P2) and −30 degrees F. (T2). Upon exiting the heat pump 28, the ethane 16 may have a pressure of 375 PSIA (P3) and a temperature of 92 degrees F. (T3). It should be understood that these values are provided merely as one non-limiting example of suitable operating parameters. Each of the pressure values may be higher or lower by five percent, ten percent, twenty percent, forty percent, or fifty percent. Each of the temperature values may be higher or lower by five percent, ten percent, twenty percent, forty percent, or fifty percent. In some instances, the pressure and temperature values may vary by more than fifty percent.

In the illustrated system 10, a propylene refrigeration unit 32 may also be used to cool the ethylene 12. The use of the ethane 16 from the C2 splitter 18 will unload the propylene compressor (not shown). Additionally, it should be noted that the heat pump 28 will provide additional reboil duty to the C2 splitter 18, which may reduce the duty on the main C2 splitter reboiler 34.

It should be appreciated that the system 10 may be retrofitted into an existing ethylene unit. That is, the line 20 may be used to draw some of the ethane 16 that is otherwise heated in a heat exchanger 34 and direct the drawn ethane 16 to a pre-existing let-down valve 22 and vaporizer 24. Thus, in a sense, the only new component is the heat pump 28 and lines 20, 30.

It should be understood that ethylene discharge gas is only one non-limiting process stream that may be cooled by the methods of the present disclosure. In other embodiments, this method may be used to cool and/or condense other fluids (i.e., gases, liquids, mixtures, etc.) in the heat exchanger 24.

While the foregoing is directed to embodiments of the present disclosure, other and further embodiments of the disclosure may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow. 

What is claimed is:
 1. A method of cooling a process stream, comprising: drawing a liquid ethane from a C2 splitter; reducing a pressure of the drawn liquid ethane in a let-down valve to produce a colder liquid ethane; cooling the process stream with the colder liquid ethane in a vaporizer, the ethane exiting the vaporizer as vapor; pressurizing the ethane vapor in a heat pump to produce a vapor ethane at higher pressure; and returning the vapor ethane to the C2 splitter.
 2. The method of claim 1, wherein the process stream is an ethylene discharge gas from an ethylene refrigerant compressor.
 3. The method of claim 1, wherein the C2 splitter separates ethylene from ethane by distillation.
 4. The method of claim 1, wherein vaporizer uses indirect heating.
 5. The method of claim 1, wherein the ethane from the C2 splitter has a pressure of P1 and a temperature of T1, the ethane from the vaporizer has a pressure of P2 and a temperature of T2, and the ethane from the heat pump at a pressure of P3 and a temperature of T3, and wherein P3 is greater than P1, P1 is greater than P2, and T1 is greater than T2.
 6. The method of claim 1, wherein: the process stream is ethylene discharge gas from an ethylene refrigerant compressor; the C2 splitter separates ethylene from ethane by distillation; the vaporizer uses indirect heating; the ethane from the C2 splitter has a pressure of P1 and a temperature of T1, the ethane from the vaporizer has a pressure of P2 and a temperature of T2, and the ethane from the heat pump at a pressure of P3 and a temperature of T3, and wherein P3 is greater than P1, P1 is greater than P2, and T1 is greater than T2.
 7. A system for cooling a process stream, comprising: a C2 splitter; a let-down valve receiving ethane from the C2 splitter and producing a colder liquid ethane; a vaporizer receiving the colder liquid ethane from the let-down valve and cooling or condensing the process stream, the ethane exiting the vaporizer as an ethane vapor: a heat pump pressurizing the ethane vapor and producing a vapor ethane at higher pressure; and a line returning the vapor ethane to the C2 splitter.
 8. The system of claim 7, further comprising an ethylene refrigerant compressor supplying the process stream, the process stream being an ethylene discharge gas.
 9. The system of claim 7, wherein the C2 splitter separates ethylene from ethane by distillation.
 10. The system of claim 7, wherein vaporizer uses indirect heating.
 11. The system of claim 7, further comprising a propylene refrigeration unit cooling the process stream.
 12. The system of claim 7, wherein the ethane from the C2 splitter has a pressure of PI and a temperature of T1, the ethane from the vaporizer has a pressure of P2 and a temperature of T2, and the ethane from the heat pump as a pressure of P3 and a temperature of T3, and wherein P3 is greater than P1, P1 is greater than P2, and T1 is greater than T2.
 13. The system of claim 7, further comprising: an ethylene refrigerant compressor supplying an ethylene discharge gas, the ethylene discharge gas being the process stream; a polypropylene refrigeration unit cooling the process stream ; and wherein: the C2 splitter separates ethylene from ethane by distillation; the vaporizer uses indirect heating; and the ethane from the C2 splitter has a pressure of P1 and a temperature of T1, the ethane from the vaporizer has a pressure of P2 and a temperature of T2, and the ethane from the heat pump as a pressure of P3 and a temperature of T3, and wherein P3 is greater than P1, P1 is greater than P2, and T1 is greater than T2. 